1. Field of the Invention
The present invention relates to a charging device which charges an image bearing member with a charging roller in an electrophotographic image forming apparatus and which has a cleaner cleaning the charging roller. In addition, the present invention also relates to an image forming apparatus, such as copiers and printers which use the charging device, and a process cartridge using the charging device.
2. Discussion of the Background
In conventional electrophotographic image forming apparatus, an image is typically formed by the following method:    (1) an image bearing member, such as photoreceptors, is charged with a charge having a predetermined polarity (i.e., charging process);    (2) the image bearing member is exposed to light to form a latent electrostatic image thereon (i.e., light irradiation process);    (3) the latent electrostatic image is developed with a toner having a charge with the same polarity as that of the latent electrostatic image to form a toner image (i.e., developing process);    (4) the toner image is transferred to a receiving material such as papers (i.e., transferring process); and    (5) the toner image is fixed on the receiving material upon application of heat and pressure thereto to form a hard copy (i.e., fixing process).
Even after the transfer process, a small amount of toner particles remains on the surface of the image bearing member. Therefore, the surface of the image bearing member is typically cleaned by a cleaner, such as cleaning blades and cleaning brushes, before the next charging process.
Recently, either contact charging methods in which a voltage is applied to an image bearing member by an electroconductive charging roller in contact with the image bearing member or short-range charging methods in which a voltage is applied to an image bearing member by an electroconductive charging roller set in the vicinity of the image bearing member, are typically used for the charging process. This is because these charging methods have advantages, such that the amount of ozone generated due to charger discharging can be controlled and the power consumption of the charger can be reduced.
However, when residual toner particles are insufficiently removed, a problem occurs in that, when the remaining toner particles contact with or are close to the charging roller, the remaining toner particles may adhere thereto. This is because the remaining toner particles typically include toner particles which have a charge with a polarity opposite to the polarity of the charging roller, thus the reversely-charged toner particles are attracted to the charging roller, resulting in adhesion of the toner particles to the surface of the charging roller. In addition, dust such as paper dust generated by receiving papers, which has a charge with a polarity opposite to that of the charging roller can also adhere to the charging roller.
Recently a need has existed for an electrophotographic image forming apparatus capable of producing high quality and high definition images. Therefore, spherical toners, having a relatively small particle diameter, are typically used to form a toner image because they can be densely adhered to a latent electrostatic image. However, one of the drawbacks of such a small spherical toner is that a cleaning blade cannot properly scrape the toner particles because often they pass through the nip between the image bearing member and the cleaning blade, resulting in the occurrence of insufficient cleaning of the surface of the image bearing member (namely, the charging roller is contaminated with toner particles). Therefore, it is necessary to clean the surface of the charging roller to prevent the occurrence of various undesirable problems.
Specific examples of such cleaning members for use in such a charging roller include sponge materials, such as polyurethane and polyethylene foams disclosed in unexamined published Japanese patent application No. 5-297690, and brush rollers disclosed in unexamined published Japanese patent application No. 2002-221883. Toners remaining on the surface of a charging roller are removed when such cleaning members are brought into contact with and abrades the surface of the charging roller. The removed matters are collected in pores inside the sponge material or between brush fibers on the brush roller. However, the amount of the unwanted toner that can be stored in such members is limited. Therefore, maintaining good cleaning performance for a long period of time remains an unresolved issue. For example, in the case of a process cartridge including a charging roller, the charging roller needs to have a useful life as long as those of other members constituting the process cartridges, each of which has a relatively long life. Therefore, a cleaning device having such a brush roller is not suitable for such process cartridges.
In addition, it is necessary for the cleaning device to remove foreign materials such as paper dust, which adhere to the charging roller.
Because of these reasons, the need exists for a long-life charging device having a cleaner which can efficiently clean materials electrostatically adhered to the surface of a charging roller.